The present invention pertains to demodulation of high speed data and more particularly to a method for continuous phase demodulation of high speed data represented by symbol sequences which contain intersymbol interference.
In modern day communications, it is desirable to achieve high data transmission rates. Further, it is desirable to achieve a 64 KBPS time division multiple access (TDMA) burst rate over a 25 KHZ UHF satellite channel, for example. Increased use of UHF satellite channels is necessary to support the increasing demand for data in the limited radio frequency (RF) spectrum. High speed data transmitted in this manner is modulated by a transmitting device and demodulated by the receiving device.
Modulation/Demodulation methods limit the amount of data throughput which may be transmitted and received. In addition, different modulation types provide different bit error rates depending on the data transmission speed of the channel. Some commonly found modulation schemes are MSK, BPSK, QPSK and CPM.
Among these modulation methods, CPM (continuous phase modulation) is best suited to achieve a high bit rate per hertz modulation/demodulation system. MSK, BPSK and QPSK are orthogonal modulation/demodulation systems. This means that they have an L=1. L is the intersymbol interference (that is, how far back in time you must go before previous symbols have no affect on the output). Modulation/demodulation systems with larger L (intersymbol interference) tend to present a higher bit error rate (BER) in their performance.
Accordingly, it is an object of the present invention to provide a symbol state trellis maximum likelihood detection method for use with continuous phase demodulation which achieves a high bit transfer rate per hertz and a relatively low bit error rate (BER) regardless of the amount of intersymbol interference.